About this video
Gravity is a fundamental, always-attractive, non-contact force between any two masses, and we model it as acting through a gravitational field. Its strength grows with the masses involved and gets weaker as the distance between the objects' centres increases. Astronauts on the ISS still feel about 80 to 90% of Earth's gravity; they seem to float because they and the station are continuously falling around the Earth.
You'll be able to
- describe gravity as an attractive, non-contact force between any two masses.
- explain how a gravitational field lets gravity act at a distance.
- describe how the strength of gravity depends on mass and on distance.
- explain why astronauts on the ISS appear to float even though gravity acts on them.
Key words
- gravity
- A fundamental force of attraction between any two masses.
- non-contact force
- A force that acts without the objects touching.
- gravitational field
- A field a mass produces in the space around it; other masses in the field feel a force.
- fundamental force
- A basic force of nature that isn't made from other forces; gravity is one.
- orbit
- The path of an object that keeps falling towards another but keeps missing it because it moves sideways fast.
Transcript
- [Instructor] You probably know that Earth's gravity keeps you and everything around you stuck to Earth, and similarly, the Sun's gravity keeps all the planets closed together in our Solar System. This, by the way, picture is not to scale, okay? But the big question is, what exactly is gravity, and what exactly produces gravity?
Is it just the planets and stars or other things can produce it too? And we could have even more questions, like for example, the astronauts inside the space station are floating. So is there gravity over there? Well, let's find out. We experience a lot of force in our day-to-day life, like friction, tension.
There are other forces like normal forces, and of course, one of them is also gravity. But guess what? Not all these forces are fundamental. In fact, most of these forces are not fundamental forces at all. All these forces are actually fundamentally electromagnetic force.
What about gravity? Gravity is another fundamental force of nature. But what exactly is it? Well, you can think of gravity as a force of attraction. It's a fundamental force of attraction between any two masses. For example, there's a force of attraction between Earth and everything else that is on Earth, like you, your friend, the house.
Anything else that you can consider, every single thing is attracted towards the Earth. And the reason they're attracted towards the Earth is because the Earth has mass, and you and your friend and everything else, they have mass. So any two masses will attract each other with the force of gravity.
Another example, the Earth and the Moon, they both have mass, and therefore the Earth attracts the Moon with the force of gravity. Similarly, the Sun and all the other planets also have mass, and therefore they attract each other, again due to a force of gravity.
But wait a second, we might have some questions over here. The force of attraction can happen at a distance, right? That means gravity is a non-contact force. You don't need to be in contact for gravity to be exerted. But how does it work? For example, how does the Earth exert a force on the Moon without ever being in contact with it?
Well, here's how we like to think about it, okay? We like to imagine that Earth produces what we call a gravitational field everywhere. This is a field. A field exists in the space around it. And when any other mass comes in contact with the gravitational field, like for example, say the Moon, it now, because of that field, experiences a force.
So the force is mediated via the gravitational field. And I'm sure you might have even more questions about what exactly is a field and stuff, but we'll not delve too deep into it. But anyways, because of this force, the Moon is literally falling towards the Earth, but it doesn't crash into the Earth because the Moon is also moving sidewards, and as a result, it ends up orbiting the Earth.
Okay, but what about the Moon? Does it also produce a gravitational field? The answer is yes. The same thing applies over here. Moon produces also its own gravitational field, and when the Earth, which is another mass, comes in contact with that gravitational field, it also experiences a force.
And that should make sense because we know that forces come in pairs. So both the Earth and Moon are gravitationally attracting each other with exactly equal but opposite forces. But the next big question could be, what does the strength of this force depend on?
Well, since gravity is a force due to masses, well, the strength depends on the mass. In fact, if both the masses are pretty large, like in the case of the Earth and the Moon, then the force is pretty large. Even if one of the objects has a very small mass but the other object has a big mass, like for example, if you consider the Earth and say a tennis ball.
In this case, the tennis ball has a very tiny mass compared to Earth, but the Earth has a such a huge mass. Even then you get a comparable force of gravity, and that's why you do see the ball falling down. But what about the gravitational force of attraction between say two tennis balls?
Remember, gravity acts between any two masses, so there must be a force of, you know, attraction between them. But we don't see it. We don't see two tennis balls getting attracted to each other. Why is that? Well, that's because both of these masses are incredibly tiny, and therefore the force of attraction between them, the gravitational force is so, so weak that there are other forces that can easily balance them.
For example, air resistance. And that's why we usually do not see the effects of gravity on, you know, everyday objects, like two tennis balls. Or maybe, you know, gravitational force between you and your friend. It's the same story. There is gravitational attraction between you two for sure.
But since the masses are so small, the forces are so tiny, again, forces like friction can easily balance them and you don't see their effects. But on the other hand, if you consider the force of attraction between you and the Earth, that's pretty large because the Earth is very massive.
Okay, so the strength of gravity depends upon the masses. At least one of the objects must have a very high mass for conceivable amount of gravity. Got it. What else does the strength of the force depend on? Well, again, if we go back to the Earth and the Moon, another thing that the force depends on is the distance between them, or to be more precise, the distance between the centers of masses of the two objects, okay?
What happens as the distance increases? Well, as the distance increases, the force actually becomes weaker. The force becomes smaller. Gravity becomes weaker as the distance between objects increases. The bigger the distance between them, the weaker the force of gravity.
So here's now a quick question. What do you think happens to the force of gravity between Earth and say you when you climb the mountain, when you go to the top of the mountain? What do you think will happen to the force of gravity between you and Earth? Why don't you pause and think about this?
All right, we might think that as we climb the mountain, the distance between us and the center of the Earth has increased. We have gone farther away from the center, so the gravity must get weaker, right? Now, technically that's true, but look at the size of the Earth and compare that to the size of the mountain.
The size of the mountain is negligible. It just looks like a dot. And therefore, for all practical purposes, climbing the mountain has not changed the distance between you and the center of the Earth, and therefore it pretty much stays the same. And that's why the force of gravity in, again, everyday life stays pretty much the same.
It does not change with height or altitude because it's very tiny compared to the radius of the Earth. But what if you go very far away? For example, what if you go as far as the International Space Station? Ooh, now you're so far away from the surface of the Earth that the distance between you and the center of the Earth has become considerably bigger, and so the force of gravity does become weaker.
But guess what? People over there still feel about 80 to 90% of the gravity they would feel on the surface of the planet. So it's become weaker, but not that weak. So the folks inside the ISS will definitely feel a significant amount of gravitational force from the Earth.
But wait a second. Why are the astronauts inside the space station floating? If they feel the force of gravity, why are they floating? Well, the short answer is they're not floating. They are falling. The whole space station is actually falling towards the Earth, giving them the illusion of floating.
But again, the reason why the space station will not crash into the Earth is because just like the Moon, it is moving sidewards with a very high velocity. So it's continuously falling towards the Earth, but keeps missing the Earth because of the sideward motion, because of the sideward velocity.
In fact, if there was no gravity, it wouldn't be orbiting the Earth. So the fact that it's orbiting the Earth is actually showing us that there is gravity over there. So, long story short, what exactly is gravity? It's a fundamental force of attraction between any two masses.
Remember, it's always attractive and its feature is that it's a non-contact force. You don't need to be in contact for that. And how does it get mediated? It gets mediated via gravitational fields. And what does the strength of the force depend on? Well, it depends on the masses, more the mass, more the force, and it gets weaker with the distance.
Well, more the distance between the two objects, between the centers of the objects, smaller is the force of gravity.
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